1. Field of the Invention
The present invention relates generally to power steering pumps and more specifically to an assembly that filters fluid directed to a relief system.
2. Description of Related Art
A power steering system used with an automotive vehicle typically utilizes a pump that provides hydraulic fluid under pressure. The pump includes a rotor having retractable vanes rotating within a cam chamber. The pump draws hydraulic fluid into the cam chamber from a fluid suction passage and pumps out the hydraulic fluid under pressure to a fluid discharge port. A belt and pulley assembly connected to the vehicle engine drives the pump. Accordingly, as the speed of the engine increases the volume of fluid pumped by the rotor also increases. It is desirable to maintain the pump output at a constant level. Accordingly, the pump includes a recycling or bypass system located in the pump housing that diverts fluid from the outlet or fluid discharge port to the suction passage.
The pump may include a mechanical valve that opens and closes the fluid bypass port to regulate pump output. The mechanical valve is a flow control valve disposed in a bore located in the pump housing. The bore communicates with and connects the fluid discharge port and the fluid bypass port. Accordingly, the flow control valve moves within the bore to open and close the bypass port. A spring located in the bore biases the valve towards a closed position. A relief system fluid flow passage extends between and provides a feedback pressure from the pump output to a pressure chamber located at or adjacent one end of the flow control valve. This feedback pressure combines with the spring force to keep the bypass port closed.
The flow control valve may include a slideable piston that engages a flow control fitting. As known, the flow from the pump exerts pressure on the piston, when the pressure exceeds the force provided by the spring, the piston will move away from the flow control fitting and the fluid will flow through the fitting to the output. If the pressure continues to increase the piston is forced further back until it uncovers the bypass port enabling excess fluid to recirculate to the pump. If the flow demand increases, the pressure will decrease and the spring forces the piston back toward the fitting to a new flow regulating position. Accordingly, the piston moves toward and away from the flow control fitting in response to changes in the pressure demand.
Occasionally contamination, either through the manufacturing process, degradation of steering system components or through system maintenance, enters the system and can clog or become lodged in the relief valve or relief system fluid flow passage of the power steering pump. This leads to higher effort on the part of the vehicle operator as the power steering system is prohibited from building pressure and thus supplies a lower amount of assistance to the vehicle operator. One manner of preventing contamination is to install a filter on the end of the relief valve. This method does not prevent contamination from becoming lodged between the valve and the valve bore. In addition, excessive contamination may cover the filter and there may be no pressure relief for the power steering system.
Therefore, a need exists for a power steering pump having a system that filters fluid directed through the relief system of the power steering pump to the relief valve and reduces the opportunity for contamination to lodge in the relief valve in a manner that reduces steering system pressure.